This invention relates to a substantially continuous method of coating an optical waveguide filament, and more particularly to a method of coating such a filament wherein it comes in contact with the coating material only within the tapered bore portion of the die. A description of glass optical waveguides may be found in U.S. Pat. Nos. 3,659,915, 3,823,995, and 3,884,550, which patents are expressly incorporated herein by reference.
Glass optical waveguides must exhibit high strength in order to withstand the stresses which are encountered in incorporating them into protective sheathing or cable, installing the cable, or in use. While such waveguides are typically quite strong as drawn from a preform or blank, this strength is rapidly degraded by surface defects which are introduced into the waveguide through handling or otherwise.
To preserve the strength of a newly drawn waveguide filament, it is conventional to apply to the waveguide immediately after it is drawn a thin protective coating composed of an organic or inorganic coating material which serves to shield the waveguide during subsequent handling. A variety of coating methods may be used for this purpose, but one common technique is to pass the filament, as it is drawn, into a reservoir of a suitable coating material and out of the reservoir through a small-bore die, often called a coating die. Reference to the use of such a coating die may be found in U.S. Pat. No. 3,980,390 to Yamamoto et al. A flexible waveguide coating die is described by Albarino et al. in U.S. Pat. No. Des. T963,002.
With recent emphasis on increasing waveguide draw speeds, attention has been directed to die design, and the use of coating dies with tapered bores has been proposed. The aforementioned flexible Albarino et al. coating dies have tapered bores, as do the rigid coating dies described by P. W. France et al., in Proc. 3rd European Conference on Optical Fiber Comm., pp. 90-92 (1977). In theory, the fluid dynamics of tapered bores give rise to forces which tend to center the waveguides in the bore, improving coating concentricity.
However, difficulties have been encountered as the filament passes through the coating solution in a tank or reservoir to the die. Turbulence is created which induces air bubbles, which air bubbles upset the alignment of the fiber in the die and the entrapped air becomes embedded in the fiber coating as the bubbles enter the die orifice. Furthermore, other complications caused by contact of the fiber and the coating solution in the tank or reservoir may create coating flaws. It has been found that air becomes entrapped in the coating material due to turbulence created by the passing filament when the filament coating velocity is in excess of approximately 0.5 m./sec. The volocity may vary, however, depending on the type of coating material used. Such embedded air bubbles may weaken the ultimate coated filament and affect its optical properties, as may a variation of coating thickness along the cross-section thereof, the fiber coating being termed "out of round" in the latter case.